1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, a facsimile, and a printer, which transfers toner images formed on a plurality of latent-image carriers, on an endless moving body such as an intermediate transfer belt or on a recording medium held on the surface of the moving body to obtain an superimposed image and a latent-image-carrier position adjusting method for the image forming apparatus.
2. Description of the Related Art
A typical color image forming apparatus includes a plurality of image forming units and successively transfers images of different colors on a recording paper through an intermediate transfer belt. In this type of image forming apparatus, latent-image carriers can be out of parallelism due to a skew in members of the apparatus depending on the accuracy of the respective components or in assembling the apparatus or a change of the temperature. In such cases, a color misalignment occurs in a sub scanning direction (a direction of movement of the surface of the intermediate transfer belt that abuts the latent-image carriers) for a color image formed by superimposing images of respective colors on an intermediate transfer belt.
In an image forming apparatus described in Japanese Patent Application Laid-open No. 2000-347474, photosensitive drums that are latent-image carriers as image forming modules of different colors, and a laser scanner unit that exposes the photosensitive drums are formed integrally. The adjacent image forming modules are contacted each other to keep a proper parallelism between the photosensitive drums, thereby suppressing a color misalignment in the color image. When the photosensitive drums are out of parallelism due to the a skew caused by a change of the temperature, a voltage is applied to a piezoelectric element that is provided between the adjacent image forming modules, based on a result of detection of toner images of different colors for detection, formed on the intermediate transfer belt from the photosensitive drums. Accordingly, the distance between the image forming modules is adjusted to correct the parallelism between the photosensitive drums.
The image forming apparatus described in Japanese Patent Application Laid-open No. 2000-347474 is based on the assumption that the accuracies of members that are produced by the same production processes and of image forming modules assembled by the same assembly processes are the same, and the same members at the same positions in the corresponding image forming modules have the same skew caused by the change of the temperature. However, even when the same production processes or assembly processes have been undergone, it is hard to achieve the same accuracy or skew associated with the image forming modules. Particularly, when the members are mass-produced, the difficulty is considerably large. Practically, for example, a longitudinal direction of the photosensitive drum and a longitudinal direction of the image forming module may not be parallel depending on the accuracies or skews of the members that configure the image forming module. In this case, an adjusted amount of a distance between the image forming modules and an adjusted amount of a position of the photosensitive drum required to correct the parallelism between the photosensitive drums have a difference. Therefore, even when the parallelism between the respective photosensitive drums is corrected by adjusting the distance between the adjacent image forming modules, like in the image forming apparatus described in Japanese Patent Application Laid-open No. 2000-347474, the parallelism between the respective photosensitive drums cannot be corrected accurately. Accordingly, the color misalignment in the sub scanning direction of a color image cannot be properly suppressed. Even when the same accuracy can be achieved in the respective image forming modules, the cost required for production of one apparatus becomes quite high.